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Ternary microemulsions of vinylic and acrylic monomers
European Polymer Journal 38 (2002) 1691–1701 www.elsevier.com/locate/europolj
Ternary microemulsions of vinylic and acrylic monomers Dan Donescu a
a,*
, Liana Fusulan a, Cristian Petcu a, Marilena Vasilescu b, Carmen Smarandache c, Ignac Capek d
Center for Plastic Material Research, Institute of Chemical Research, ICECHIM, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania b Institute of Physical Chemistry, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania c Institute of Organic Chemistry, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania d Polymer Institute, Slovak Academy, Dubravska cesta 9, 842 36 Bratislava, Slovakia Received 3 April 2001; received in revised form 10 December 2001; accepted 17 January 2002
Abstract Ternary systems consisting of sodium dodecyl sulfate (SDS) as surfactant, water and several vinyl and acrylic monomers [vinyl acetate (VAc), acrylonitrile (ACN), ethyl acrylate (EtA), butyl acrylate (BuA), 2-ethylhexyl acrylate (EHA), methyl methacrylate (MMA), butyl methacrylate (MMB) and styrene (St)] were studied. The solubilization of monomer in aqueous solutions of SDS was found to be dependent on its structure and concentration. The molar specific solubility was observed to decrease with hydrophobicity and increase with polarity of monomer, that is, it was lowest for St, EHA and highest for MMA, EtA. The NMR and fluorescence studies indicate that solubilization occurred at a different domain of the interfacial layer. The hydrophobic monomers are solubilized toward the hydrocarbon interior of the micelles whereas the hydrophilic ones, toward the hydrated tail of the surfactant. The penetration of monomers into the oil-in-water interface is limited because the screening of charged ions of emulsifier is not operative. A relationship between the persulfate initiator decomposition rate and the lability of the a-hydrogen linked to the substituted carbon of the double bond was established. The initiator productivity was the highest for MMA (lacking such a-hydrogen) and the lowest for VAc and St, the monomers in which the C–Ha bond is the most reactive. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Microemulsions; SDS; H2 O; Vinylic, acrylic monomers; Solubilization; Polymerization
1. Introduction Oil-in-water (o/w) microemulsion consists of oil droplets (10–20 nm) dispersed in water with the aid of emulsifier and coemulsifier [1]. A mixture of water, oil, and amphiphile can form a thermodynamically stable, optically transparent one-phase solution (termed the microemulsion). The hydrophobic and electrostatic forces among the ionic emulsifier molecules play an essential role for the self-association and formation of micelles. One of the most interesting aspects of these
* Corresponding author. Tel.: +40-1-315-3299; fax: +40-1312-3493.
micelles is their ability to accommodate oil-soluble molecules. The increased flexibility of the micellar membrane and, as a result, the improved capability of micelles to solubilize hydrophobic molecules can be achieved by adding a coemulsifier [2]. In an ordinary o/w emulsion, the oil is located in three phases: the emulsified oil droplets, the continuous aqueous phase and the interiors of oil-swollen micelles. Once such oil droplets are present, any further oil added leads only to the formation of more or larger oil droplets, since the aqueous phase is saturated with oil. There exists a thermodynamic limit, called the ‘‘solubilization limit’’, to the amount of oil that can be taken up by the micelles. Systems with compositions below the solubilization limit are in fact examples of thermodynamically
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 2 9 - 0
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stable microemulsions, whether coemulsifier is present or not [3]. Microemulsions above the solubilization limit are kinetically stable, or more correctly miniemulsions. Systems above the solubilization limit achieve thermodynamic equilibrium by separating into two phases [4]. A knowledge of the solubilization limit of a monomer in the micellar aggregates is of considerable importance in interpreting the polymerization kinetics. The solubilization of the monomers in the surfactant–water mixture depends on the chemical structure of the components [5– 10]. Studies by NMR spectroscopy performed with ternary (emulsifier, water and monomer) mixtures revealed the fact that the monomers are distributed in the whole micellar volume [11,12]. Depending on its polarity, the monomer interacts either with the internal hydrocarbon part or with the external polar side of the surfactant [11,12]. Pyrene (Py) as a probe is particularly attractive because of its relatively long fluorescence lifetime, its ability to measure the polarity of its microenvironment, and its tendency to form excimers with a distinct fluorescence [13]. Thus, Py is a useful indicator of the polarity via the ratio of intensities of the first to the third peaks in the fluorescence emission spectrum of Py, I1 =I3 and it is widely used to probe the microenvironment of organized host media such as micelles [14]. In water, I1 =I3 is equal to 1.7, whereas in a nonpolar solvent it has a value well below 1 (e.g., in cyclohexane, I1 =I3 ¼ 0:58 [15]). In ionic (SDS) and nonionic emulsifier micelles, I1 =I3 equals 1.1–1.3, indicating that the probe environment is hydrophobic, but some contact with water still remains [16,17]. In micellar systems there is always a fraction of the probe molecules (free emulsifier molecules, oil-phase, etc.) that is in a dynamic equilibrium with that in the micellar phase. It was reported [17] that in the micellar solutions, as much as 5% of pyrene molecules remain in the aqueous phase (½Pytotal ¼ 30 lM). Furthermore, the excimer-to-monomer ratio IE/IM is a characteristic of the system that can serve as an indicator of a local concentration of a probe in the micelles. When plotted against ½Py=½E, the excimer-tomonomer ratio is a characteristic feature of a micellar system, provided that the size and aggregation number do not change. [E] represents the concentration of emulsifier. It was, however, reported that I1 =I3 increased from about 1.7 in pure water to about 2 in polyoxyethylene/ water (20/80, w/w) mixture [18]. It was concluded that that the increase in the I1 =I3 ratio upon complexation with n-dodecyl hexaethylene glycol monoether/n-hexadecyltrimethylammonium chloride mixed micelles was an indication that Py labels were solubilized in the hexa(oxyethylene) layer in the micelle. It is known that the pyrene spectrum is strongly affected by the polarity of the microenvironment [16,19]. In micellar solutions of SDS, pyrene can also be bounded to the hydrated ethylene groups close to the sulfate tails [16,19].
The polymerization kinetic order of the initiator is also dependent on the structure of the monomer in the polymerization occurring in ternary microemulsions [2,5–12,20,21]. An increase in decomposition rate of persulfate initiators was observed in ternary emulsions, ascribed to abstraction of a-proton from the substituted double bond of the monomer [22]. This effect was also found in microemulsions using alcohols as cosurfactants [23]. In the present paper, the solubilization of several monomers in the mixture water–sodium dodecylsulfate (SDS) was studied in order to find the selectivity of solute/surfactant interactions. Studies by NMR spectroscopy were performed to provide evidence for the distribution of the monomers in micellar aggregates. Fluorescence spectra were also used to gain more information on the solubilization zone of the monomers. Because it is well known that the decomposition reaction of the persulfate initiator is affected by the interaction with the monomers, the initiator decomposition and the polymerization kinetics were studied. The initiator productivity was estimated by taking into account the two simultaneous radical processes, namely the polymerization of the monomers and the initiator decomposition.
2. Experimental 2.1. Materials The monomers: vinyl acetate (VAc), acrylonitrile (ACN), ethyl acrylate (EtA), butyl acrylate (BuA), 2ethylhexyl acrylate (EHA), methyl methacrylate (MMA), butyl methacrylate (MMB), styrene (St) were commercial products and were fractionated before use. The surfactant, sodium dodecylsulfate (SDS) (>99%, Merck) and the initiator, ammonium persulfate (APS) (Loba Feinchemie) were commercial grade. 2.2. General procedure The polymerizations were performed in a glass reactor equipped with mechanical stirring (200 rpm). The reagents were added under stirring in the order: water, SDS and monomer. The mixture was degassed for 1 h with argon and heated up to 65 °C, when the initiator APS was added. Table 1 displays the concentration of the reactants. The solubilizations were performed by titration of surfactant solutions with monomers at 25 °C (Fig. 1). Due to the high viscosity of the SDS solutions, the maximum SDS/water ratio was 25/75 (weight). The solubilization capacity was calculated taking into account the amount of monomer soluble in water (Fig. 2).
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2.3. Analyses
Table 1 Nature and quantity of the reagents Reactant (g)
I
II
Monomer SDS APS H2 O Total
15 42.5 0.75 242.5 300.75
3 6 0.75 291 300.75
III 3 – 0.75 297 300.75
The 1 H-NMR spectra were recorded with a Brucker Avance DRX 400 instrument operating at 400 MHz. The chemical shifts of a-vinylic protons were determinated for 10% CDCl3 solutions (dI ) as well as in 15% SDS for D2 O solutions (dII ). The fluorescence measurements were performed with a Shimadzu RF-5001PC instrument, using pyrene
Fig. 1. Phase diagrams for SDS, water, monomers (max SDS=H2 O ¼ 25=75; 25°C).
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transparent mixtures obtained by solubilization of monomers were o/w microemulsions consisting of micellar aggregates swelled by monomers [9]. Similar one-phase microemulsion regions of monomer/water/surfactant system were obtained by other authors [5–10]. Ternary mixtures within the microemulsion region are transparent, fluid except at high SDS concentration (>25% SDS) where they are transparent and highly viscous. Using the results given in Fig. 1 we estimated the solubilization behaviour (Fig. 2). The molar ratios between solubilized monomer and surfactant were calculated by comparing the specific solubilization capacities of micelles (Fig. 2). Solubilization capacities between 0.5 and 3.5 mole monomer/mole SDS were found; the lowest values corresponding to the less polar St, EHA and the highest values for the more polar EtA and MMA. The polar monomers MMA and EtA gave the highest amount of solubilized monomer at the highest SDS concentration. The monomers presenting specific solubilization capacities independent of SDS concentration (ACN, VAc) have the highest solubility in water [26]. Furthermore, an increase in the solubilization of monomers appears with BuA and MMA. The synergistic effect of BA as a coemulsifier increased with SDS concentration [27]. For example, at the mole ratio BuA/SDS, 0.25/0.25, the coemulsifier efficiency of BuA was comparable with that of 1-pentanol (classical coemulsifier). Under the circumstances, the transparent microemulsion with the average droplet diameter 4.5 nm was formed. At lower SDS concentration, a coarse monomer emulsion or phase separation appears. The addition of styrene led to a strong increase in the droplet size. This was attributed to the different location of BA and St. It is most likely that the difference in the solubilization activity of both monomers is due to the different coemulsifier activities between St and BuA. Guo et al. [28] proved that the partitioning behaviour of styrene and 1-pentanol (POL) are different. The decreasing in POL concentration is as follows: interfacial layer oil > aqueous phase. Furthermore, the partitioning of hydrophobic styrene is modified in the
Fig. 2. The modification of the solubilization of the monomers versus SDS concentration.
(5 106 M) as fluorescence probe. The fluorescence spectra were recorded with pure monomers and with their solutions in SDS/water mixtures in concentrations similar to those used in the NMR measurements. The I1 =I3 ratios in the fluorescence spectra are given in Table 2. The monomer conversion was established gravimetrically. The APS conversion was established by ceriometric titration of the unreacted initiator [24]. The particle diameters of the latexes were determined with a NICOMP 270 instrument [25].
3. Results and discussion 3.1. Solubilization of monomers 3.1.1. Phase diagrams of microemulsions The ternary diagrams of mixtures of monomers, SDS, water are given in Fig. 1. The solubilization capacity of the monomers can be related to their polarity: the more polar EtA, MMA, VAc, ACN were solubilized in higher concentration than the less polar St, EHA, MMB. The
Table 2 Parameters of the fluorescence spectra of the pyrene in the presence of the monomers and of the microemulsions 15% SDS
MMA
EHA
BuA
CAN
St
EtA
VAc
0.807 1.019 1.132
0.92 0.931 –
1.055 1.055 –
1.137 1.087 1.038
1.273 1.078 –
2.61 (excimer) 1.611 (excimer) 1.675 (excimer)
2.931 (excimer) 1.041 1.069
1.03 1.19 1.73
1.74 1.17
1.23 1.22
0.91 2.77 3.46
2.86 8.63
3.83 (excimer) 0.84 (excimer) 1.08 (excimer)
6.9 (excimer) 6.69 5.33
I1 /I3 1.072 5% mon þ 5% SDS 10% mon þ 5% SDS 104 Intensity I1 10.22 5% mon þ 5% SDS 10% mon þ 5% SDS
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order: oil interface > water. The increased incorporation of monomer into the interfacial layer increases the solubilization capacity of the micellar aggregates. Thus, the fraction of BuA, MMA and EtA incorporated in the interfacial layer is much larger that that of styrene. Incorporation of an ‘‘amphipathic’’ additive (MMA, BuA, etc.) into the adsorbed layer of SDS around a monomer droplet reduces the electrostatic repulsion force between two SDS molecules, minimizes the oil–water interfacial tension and increases the flexibility of the interfacial layer as well as its total surface area. Under the circumstances, the spontaneous formation of transparent one-phase microemulsions can occur. Guo et al. [28] reported that the mole ratio of POL to SDS at the oil–water interface is 0.9 (the initial mole ratios: ½SDS=½POL ¼ 0:7 and ½SDS=½St ¼ 0:65). Chern and Liu [29] reported for POL 1.1 and hexanol 0.7 (with SDS), that is, the transition of macroemulsion to microemulsion starts earlier with more hydrophobic coemulsifier and the interfacial layer is more saturated with (hydrophilic) POL than with (hydrophobic) hexanol. Furthermore, the hydrophile–lipophile balance (HLB) for butanol, pentanol and hexanol was estimated to be 7, 6.53 and 6.05, respectively. The HLB data for BuA, EtA or MMA are expected to be much smaller than 6–8. Therefore, the alkyl (meth)acrylate monomers tend to adsorb into the microemulsion droplet surface layer (its inner part) or their penetration into the interfacial layer is somewhat depressed. 3.1.2. NMR spectroscopy Additional information on solubilization of monomers in SDS aggregates were obtained from 1 H-NMR spectra, observing both the signal of the surfactant and monomer. It was reported that the SDS 1 H-NMR proton spectrum has four peaks assigned SDS1, SDS2, SDS3, and SDS4 [30]:
SO4 –CH2 –CH2 –ðCH2 Þ9 –CH3 SDS1
SDS2
SDS3
SDS4
The chemical shifts of the CH3 (SDS4) group (hydrophobic head) and the methylenic protons in the CH2 OSO 3 group (SDS1) (hydrophilic site) of the surfactant were determined in the absence (d1 ) and in the presence (d2 ) of the monomer. The ratio (r) between DdCH3 =DdCH2 [11,12] calculated for the CH3 signal and corresponding dCH2 calculated for the CH2 (SDS1) signal was taken as a relative measure of the monomer location in the interfacial layer. It is represented in Fig. 3 for each monomer, at two different concentrations of monomers namely 5% and 10%. These data show each monomer located at a different domain of the interfacial layer. The highest r value obtained for hydrophobic St monomer indicates that monomer locates in the inner part of the interface (the lamelar layer of SDS). In the series of more polar monomers such as EtA, BuA, EHA
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Fig. 3. The variation of the ratio DdðCH3 Þ=DdðCH2 Þ with monomer structure for 5% and 10% weight conc. of monomers.
and MMA, MMB, a steady decrease of the r value (the shift to the polar part of the interfacial layer) was observed with increasing hydrophobicity or alkyl chain length of the alkyl ester group of the monomer and the decrease is more pronounced with alkyl acrylates: EtA 6 MMA < BuA ¼ EHA < MMB This indicates that the presence of a polar group in a monomer molecule (its hydrophilic part) favors the shift of monomer within the outer (polar) part of the interfacial layer. Furthermore, r increases with the monomer concentration due to the increased penetration of monomer into the interfacial layer (up to the saturation limit). The values of r indicate that solubilization of the above monomers occurred preferentially in the hydrophobic site of the micelles. This means that the incorporation of monomer does not screen the negatively charged ion of emulsifier, the necessary condition for the formation of thermodymanically stable microemulsion with the high fraction of solubilized oil. The hydrophilic VAc gave the lowest r value, in agreement with its shortest hydrocarbonate moiety. The value r ¼ 0:4–0:5 indicates that the domain of location of VAc within the interfacial layer is shifted to its more polar part but not to the charged part of the emulsifier. Indeed, the formation of an unstable micellar solution at higher VAc concentration does not favor the screening effect of VAc. On the contrary, the hydrophilic ACN monomer gave a very high r value. However, while all other monomers gave positive DdCH3 and DdCH2 values, ACN gave negative Dd values for both target groups. This peculiar behavior of ACN has been previously reported and was explained by its high polarity and its capacity to make loose types of micellar aggregates [11,12]. The 1 H-NMR signal of vinylic protons at the substituted carbon atom were recorded in CDCl3 with a 10% concentration of monomer (dI ) and in D2 O solution with
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Fig. 4. The differences (dII dI ) for monomers solubilized in SDS solutions (dII ) and in CDCl3 (dI ).
15% SDS for two concentrations of monomer, namely 5% and 10% (dII ). The chemical shift difference Dd ¼ dI dII is represented in Fig. 4. All signals except one for ACN moved downfield with increasing the hydrophobicity and concentration of monomer. It is well known that micelle formation, alcohol addition, and salt addition to micellar solutions of ionic emulsifiers are accompanied by downfield shift of all carbon resonances of emulsifier alkyl chains. The downfield shift is due to an increased fraction of trans conformers around the C–C bonds [31]. Elongated emulsifier chains have a greater population of trans conformers than contracted emulsifier chains. Increased aggregation of emulsifier molecules by addition of salt or alcohol causes closer packing of monomers and elongation. Thus, the downfield shift is attributed to increased incorporation of monomer into the interfacial layer due to which the shielding of vinylic protons of monomer is decreased. An increased downfield shift (Dd value) along with increasing length of the hydrocarbonate moiety of the acrylic monomers or hydrophobicity of monomer observed (EtA < BuA < EHA < St) is in good agreement with the previous results presented in Fig. 3. Elongated monomer alkyl monomer chains within the interfacial layer have a greater population of trans conformers. Thus, the conformation freedom increases from EtA to St. Polar molecules which are closer to the emulsifier polar head experience less conformation freedom (more trans conformers) than those further away. As monomer is forced away from the polar groups of emulsifier, the downfield shift appears. Thus, the least polar (or highest hydrophobic) monomer, St, gave the highest Dd value. In the case of St, the strong downfield shift (increased conformation freedom) can result from the decreased conjugation caused by the close packing of St within the lamellar phase of SDS. Furthermore, the results indicate that location of the monomer in the micellar solution
occurred at sites of lower polarity than in CDCl3 solution and these are the hydrophobic lamellar phase of the surfactant. The hydrophilic VAc monomer deviates from this trend, that is, the Dd value is relatively large. The Dd value for VAc is larger than that for BuA (the reverse was expected) and the shift is nearly independent of VAc concentration. The results indicate that VAc is located in a relatively hydrophobic environment. The independence of Dd on VAc concentration results from the saturation of interfacial layer reached already at lower monomer concentration. The high water solubility of VAc and the partitioning monomer between the micelles and the continuous phase cannot be ruled out. The fraction of the relatively hydrophilic VAc dissolved in the aqueous phase cannot be ignored and, consequently, the volume fraction of the aqueous phase decreases with increasing VAc concentration. This dilution effect together with the decreased dielectric constant of the aqueous phase influence by a complex way the micellar aggregation, solubilization and interaction between emulsifier (micelle) and monomer. Again, the ACN monomer had a different behavior: it gave a positive Dd value (upfield shift), meaning that bounding probably occurred at the polar site of SDS molecule or micelle (the hydrated tails of the surfactant) due to the decreased conformation freedom of monomer and/or the formation of (non)micellar clusters between emulsifier and ACN. Both decreased the conformation freedom of vinylic protons. This shift reveals a loosening or contraction of the monomer as the micellar solution is diluted by ACN. At very low ACN concentration the mean curvature of the emulsifier interface is toward oil since both oil (the hydrophobic chains of emulsifier) and ACN are contained within the aggregates and water is the continuous medium. A further addition of ACN can change the curvature toward to water, causing ACN (saturated with water) to become the continuous medium (the reverse micelles). This also might lead to the reverse trends (the high solubilization and upfield shift). 3.1.3. Effect of monomers on the fluorescence of pyrene Fluorescence spectra were also used to gain information on the solubilization zone, using pyrene as a fluorescence probe. The results of the fluorescence study for monomers and microemulsions are presented in Table 2. The I1 =I3 value for the aqueous solution of SDS (1.072) is in a good agreement with the literature result (1.1) [16,17]. As expected, the I1 =I3 value for organic solvents or monomers, MMA, EHA, BuA, ACN and St is smaller than that for pure water (1.7). The value of the I1 =I3 ratio obtained for St is that corresponding to aromatic solvents. The rest of the monomers present a polarity lower than that of aromatic solvents, yet higher than that for hydrocarbon solvents [16]. The I1 =I3 values
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for EtA and VAc, however, are higher than that for the pure water. This indicates that the probe is located in a very polar domain probably formed by association of monomer molecules. Similar results were observed in a polyoxyethylene (POE)/water mixture where Py was solubilized in the polar POE layer [18]. The accumulation of Py within the solvent clusters can explain the appearance of emission spectrum of the pyrene excimer. The formation of inverse micellar structures is not ruled out. The emulsification of a monomer solution of Py with the aqueous SDS solution, led to the change in the I1 =I3 value of monomer, that is, it increased with MMA, decreased with ACN and St and remained constant with BuA and EHA. Furthermore, the I1 =I3 values for solubilized MMA, EHA, BuA, ACN and St with SDS micelles are very close to that for the pure SDS micelles. This indicates that the lamellar phase formed hydrophobic alkyl chains of SDS which solubilizes both Py and monomer as well. Furthermore, the solubilized monomer seems to change slightly the environment of Py within the micellar aggregates. The hydrophobicity of the lamellar phase is lower than that of bulk styrene. This indicates that the probe environment within the micelles is hydrophobic, but some contact with water still remains [16]. The absence of an excimer indicates that Py molecules are homogeneously distributed within the micelle and shielded with hydrophobic alkyl chains and diluted by monomer. This is due to the location of both monomer and Py within the lamellar phase of SDS. The lowered I1 =I3 found with EHA is due to partitioning of monomer as well as Py between micelle shell and core or the formation of a distinct monomer core. The presence of the excimer in the spectra of a EtA microemulsions (Table 2) can be attributed to the presence of Py within the continuous phase (the monomer bulk contribution, see above) as well as in the monomer core. However, the decreased I1 =I3 value in the presence of SDS micelles results from the solubilization of a certain fraction of monomer. Furthermore, the presence of emulsifier or micelles is expected to change the original structure of the continuous phase as well as the value I1 =I3 . The accumulation of EtA as well as Py within the micelles is expected to change the I1 =I3 value close to 1.1 typical for SDS micellar systems. However, the average I1 =I3 value 1.6 indicated that the continuous phase is also operative. The experimental results show that the I1 =I3 value is nearly independent of EtA concentration. This can result from the competition between the micelle (decreases the average value) and the continuous phase (increases the average value) contributions. The appearance of a Py excimer can be attributed to the decreased solubility of Py in the VAc and therefore increased accumulation of Py molecules into the Py domains. The large I1 =I3 value (2.9) indicates a very
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polar probe environment. The solubilization of VAc in the SDS micelles led to an abrupt decrease in the I1 =I3 value. Furthermore, the excimer observed in bulk VAc phase disappeared by the addition of an aqueous phase solution of SDS. This is due to the solubilization of VAc as well as Py in the micellar phase. The I1 =I3 values 1.04– 1.07 favor the location of Py and VAc in the micellar phase. This is the reason why the excimer is missing in the fluorescence spectra obtained with solubilized VAc (Table 2). According to the foregoing data, VAc was solubilized in the polar zone and the pyrene is displaced towards the nonpolar lamellar site of SDS. Indeed, the I1 =I3 value with VAc is very close to that with pure SDS micelles. Table 2 indicates that the fluorescence intensity (I1 ) varies with the monomer type and reaction media. The largest I1 value is observed in the pure aqueous solution of SDS. The much lower I1 values are observed in the bulk monomer phase. This can be attributed to the fluorescence quenching by monomer [32,33]. The fluorescence quenching is stronger with MMA, EHA, BuA than with St, EtA and VAc. The addition of monomer to the aqueous solution of SDS led to very strong quenching and the quenching rate decreases in the following order: EtA > EHA P MMA P BuA > ACN > VAc > St The strong quenching caused by alkyl (meth)acrylate results from accumulation of Py (fluorescence probe) and quencher (monomer) at the same reaction loci (the micelle). The decreased quenching with ACN and VAc can be attributed to the different location of Py and quencher. The abrupt decrease in quenching efficiency of St in the micellar solutions may be discussed in terms of the conformation change of St within the close-packing lamellar phase of emulsifier. Furthermore, the increasing quencher (VAc) concentration slightly increases the quenching events. The reverse is true for MMA and ACN and EtA micellar solutions where the quenching decreases with increasing quencher concentration. This can be attributed to partitioning of monomer between the micelle and the aqueous phase and the shift of the quenching from the micelle to the bulk phase which is less efficient.
3.2. Relationship between polymerization kinetics and decomposition of the initiator In a previous study [23] it was shown that monomer structure influences not only the polymerization rate but also the decomposition of the initiator [22]. It was evidenced the possibility of extracting of one proton from substituted carbon atom by the persulfate initiator [22,23].
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In the present work, three different polymerization mixtures were used in order to obtain kinetic data for both homolytic reactions mentioned above (Table 1). In the first two runs, the effects of SDS concentration on the rate of polymerization and decomposition of APS were investigated (microemulsion polymerization). In run III, the polymerization and decomposition of APS were investigated under homogeneous reaction conditions in water. Fig. 5 lists the variation of amounts of both formed PVAc and decomposed APS with the reaction time. It is obvious that the initial decomposition of APS is very fast and becomes smaller at high conversion. The initial decomposition rate of APS (APS g/min) and the initial polymerization rate (g pol/min) increases in the following order: 0:002 ðg APS=minÞ ðrun IIIÞ > 0:0023 ðrun IÞ > 0:0025 ðrun IIÞ and 0:09 g=min ðrun IIIÞ < 0:16 ðrun IIÞ < 0:46 ðrun IÞ These results show that the initial decomposition rate is increased by the addition of SDS and the increase is
proportional to the SDS concentration. This is in a good agreement with the literature data where the addition of monomer, emulsifier and additives increase the decomposition rate of APS [34–37]. Tauer et al. [38] reported that the addition of St to an aqueous solution of APS strongly increased the decomposition rate of initiator. The maximum rate of APS decomposition increases in the following order (run I) (Fig. 6): MMA < EtA < BuA < EHA < SDS=water < VAc < ACN St In the run II, the maximum rate of APS decomposition increases in the following order: MMA < ACN < EtA < SDS=water VAc The similar trend is observed in pure water (run III): Water < MMA < EtA < ACN VAc The trend of APS decomposition rate (run I) is paralleled with the rate of polymerization in the SDS/alcohol/ monomer microemulsion initiated by APS; that is, the polymerization was fastest with VAc and St while slowest with MMA [24]. The compartmentalization of
Fig. 5. The variation in time of the polymerized VAc and of the decomposed APS (conds. as in Table 1) (I––42.5 g SDS, 15 g VAc; II––6 g SDS, 3 g VAc; III––0 g SDS, 3 g VAc; 0.75 g APS).
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Fig. 6. The modification of the initiator decomposition rate with the structure of the monomers (conds. as in Table 1).
reaction loci (run I) leads to the abrupt increase of the polymerization rate. The increased bimolecular termination (run III) decreases the polymerization rate. The comparison between the two simultaneous radical processes, namely the polymerization and the APS decomposition, was based on the initiator productivity P (Fig. 7). It was measured by the polymerized monomer (in moles) for 1 mole decomposed APS. In all cases, the monomer conversion was 50%. The P value was the lowest for VAc and St, the monomer inducing the most rapid decomposition of APS. On the contrary, MMA, which gave the slowest APS decomposition rate, had the highest P value (Fig. 7). This fact suggests that there is a secondary reaction consuming the initiator. Based on previous studies [22,23], these side reactions could be the extraction of the a-proton in the monomer. The MMA, lacking this a-proton, gave the highest P value. The extraction of any other protons from MMA should be much slower processes. Morris and Part [39] had shown that only perfluorurated compounds did not accelerate the decomposition of the persulfate initiator. The molecular mechanics and reactivity calculations gave the lowest energy value of the @C–Ha bond for St and VAc. At the same time, the free radicals corresponding to Ha extraction had the highest stability [23].
For estimating the Ha lability, the chemical shifts in the 1 H-NMR spectra were measured [23]. The mean d value of Ha determined in D2 O solutions with 15% SDS and 5% monomer decreased in the order: dmed ðppmÞ ¼ 7:19ðVAcÞ; 6:44ðStÞ; 6:099ðEtAÞ; 6:052ðBuAÞ; 6:033ðEHAÞ; 5:8ðACNÞ The above order corresponds to decreasing APS decomposition rate order, except for ACN. It could be explained by higher concentration of ACN in the aqueous phase (the solubilization site of APS) as well as to the increased lability of the protons in the ACN molecule due to the electronic effects of the CN group. The persulfate initiator gave higher polymerization rate and better stability of the latexes than benzoyl peroxide. These observations are in agreement with previous results in the literature [2]. A very good stability of the final mixtures obtained under conditions described above was obtained for VAc, St and EHA. The rest of the monomers gave latexes presenting particles agglomeration and phase separation in time. The mean diameter of the particles in the microlatexes obtained with St and EHA was 18 and 20 nm, respectively. The system obtained from VAc was transparent and stable; it gave precipitation on dilution in
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Fig. 7. The variation of the productivity of APS (P) for different monomers (P ¼ moles monomer polymerized=1 mole APS decomposed, conds. as in Table 1).
water. This is due to complex formation between PVAc and SDS [40,41].
4. Conclusion The solubilization of vinylic and acrylic monomers in water solutions containing SDS is strongly dependent on the nature of the monomer. Both 1 H-NMR and fluorescence studies agreed on solubilization occurring in the whole micellar aggregates. The hydrophobic monomers are solubilized towards the hydrocarbon interior of the micelles, whereas the hydrophilic ones, towards the head-group of the surfactant. The monomers induced different decomposition rates of the persulfate initiator. This fact was related to lability of the C–Ha bond in the monomer. The MMA, lacking the a-proton for double bond, gave highest productivity for the initiator decomposition.
References [1] Schulman JH, Riley DP. J Colloid Sci 1948;3:383. [2] Capek I. Adv Colloid Interf Sci 1999;80:85. [3] Friberg SF. J Disp Sci Technol 1985;6:317.
[4] Lyons CJ, Moore BO, Coller BAW, Wilson IR. J Disp Sci Technol 1990;11:125. [5] Xu X, Zhang Z, Wu H, Ge X, Zhang M. Polymer 1998;39:5245. [6] Dreja M, Tieke B. Langmuir 1998;14:801. [7] Loh S, Gan LM, Chew CH. J M S Pure Appl Chem 1995;A32:1681. [8] Gan LM, Chew CH, Lee KC, Mg SC. Polymer 1993;34:3064. [9] Puig JE, Perez-Luna VH, Perez-Gonzaly M, Macias ER, Rodriguez BE, Kaler EW. Colloid Polym Sci 1993;271: 114. [10] Rodriguez-Guadarrama LA, Mendizabal E, Puig JE, Kaller EW. J Appl Polym Sci 1993;48:775. [11] Konovalova VP, Iablonski OP, Beliaev AV, Tsailingold VL. Koll J 1983;45:333. [12] Iablonski OP, Konovalova VP, Beliaev AV, Tsailingold VL. Koll J 1982;44:391. [13] Rangarajan B, Coons LS, Scranton AB. Biomaterials 1996;17:649. [14] Bohorquez M, Petterson LK. Langmuir 1990;6:1739. [15] Dong DC, Winnik MA. Can J Chem 1984;62:2560. [16] Kalyanasundaram K, Thomas JK. J Am Chem Soc 1977;99:2039. [17] Vorobyova O, Yekta A, Winnik MA, Lau W. Macromolecules 1998;31:8998. [18] Mizusaki M, Morishima Y, Yoshida K, Dubin PL. Langmuir 1997;13:6941. [19] Balles BL, Almgren M. J Phys Chem 1995;99:15153.
D. Donescu et al. / European Polymer Journal 38 (2002) 1691–1701 [20] Capek I, Juranicova V, Barton J, Asua JM, Ito K. Polym Int 1997;43:l. [21] Capek I, Juranicova V. Eur Polym J 1998;34:783. [22] Litt MH, Chang KHS. In: Elasser MS, Vanderhoff JW, editors. Emulsion polymerization of vinyl acetate. Appl Sci Publ; 1981. p. 137. [23] Donescu D, Fusulan L, Vasilescu M, Donescu A, Chiraleu F, Petcu C. J Disp Sci Technol 2001;22:231. [24] Koltoff IM, Carr EM. Anal Chem 1953;22:298. [25] Donescu D, Teodorescu M, Fusulan L, Petcu C. J Disp Sci Technol 1999;20:1085. [26] Barton J. Makromol Chem Makromol Symp 1990;35/ 36:41. [27] Capek I, Liou CW, Chern CS. J Chin Inst Chem Engrs 2001;32:327. [28] Guo JS, El-Aasser MS, Sudol ED, Yue HJ, Vanderhoff JW. J Colloid Interf Sci 1990;140:175. [29] Chern CS, Liu CW. Colloid Polym Sci 2000;278:329.
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[30] Gjerde MI, Nerdal W, Hoiland H. J Colloid Interf Sci 1996;183:185. [31] Bjorling M, Karlstrom G, Linse P. J Phys Chem 1991;95:6706. [32] Barton J, Capek I, Hrdlovic PH. J Polym Sci Polym Chem Ed 1975;13:2671–90. [33] Capek I, Huang CC, Wu PL, Liaw DJ. J Polym Sci Part B 2001;39:101. [34] Kolthoff IM, Muller IK. J Am Chem Soc 1951;73:3055. [35] Alexander AE, Napper DH. Prog Polym Sci 1970;3:145. [36] Capek I, Potisk P. J Polym Sci Polym Chem 1995;33:1675. [37] Vaskova V, Hlouskova Z, Juranicova V, Barton J. Macromol Chem 1990;191:717. [38] Tauer K, Deckwer R, Kuhn I, Schllenberg C. Colloid Polym Sci 1999;77:607. [39] Moris LE, Part AG. Macromol Chem 1968;119:212. [40] Edelhauser HA. J Polym Sci Part C 1969;271:291. [41] Gosa K, Donescu D. Mat Plast 1988;25:74.
Ternary microemulsions of vinylic and acrylic monomers Dan Donescu a
a,*
, Liana Fusulan a, Cristian Petcu a, Marilena Vasilescu b, Carmen Smarandache c, Ignac Capek d
Center for Plastic Material Research, Institute of Chemical Research, ICECHIM, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania b Institute of Physical Chemistry, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania c Institute of Organic Chemistry, 202 Splaiul Independentei, PO 15/159, 77208 Bucharest 6, Romania d Polymer Institute, Slovak Academy, Dubravska cesta 9, 842 36 Bratislava, Slovakia Received 3 April 2001; received in revised form 10 December 2001; accepted 17 January 2002
Abstract Ternary systems consisting of sodium dodecyl sulfate (SDS) as surfactant, water and several vinyl and acrylic monomers [vinyl acetate (VAc), acrylonitrile (ACN), ethyl acrylate (EtA), butyl acrylate (BuA), 2-ethylhexyl acrylate (EHA), methyl methacrylate (MMA), butyl methacrylate (MMB) and styrene (St)] were studied. The solubilization of monomer in aqueous solutions of SDS was found to be dependent on its structure and concentration. The molar specific solubility was observed to decrease with hydrophobicity and increase with polarity of monomer, that is, it was lowest for St, EHA and highest for MMA, EtA. The NMR and fluorescence studies indicate that solubilization occurred at a different domain of the interfacial layer. The hydrophobic monomers are solubilized toward the hydrocarbon interior of the micelles whereas the hydrophilic ones, toward the hydrated tail of the surfactant. The penetration of monomers into the oil-in-water interface is limited because the screening of charged ions of emulsifier is not operative. A relationship between the persulfate initiator decomposition rate and the lability of the a-hydrogen linked to the substituted carbon of the double bond was established. The initiator productivity was the highest for MMA (lacking such a-hydrogen) and the lowest for VAc and St, the monomers in which the C–Ha bond is the most reactive. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Microemulsions; SDS; H2 O; Vinylic, acrylic monomers; Solubilization; Polymerization
1. Introduction Oil-in-water (o/w) microemulsion consists of oil droplets (10–20 nm) dispersed in water with the aid of emulsifier and coemulsifier [1]. A mixture of water, oil, and amphiphile can form a thermodynamically stable, optically transparent one-phase solution (termed the microemulsion). The hydrophobic and electrostatic forces among the ionic emulsifier molecules play an essential role for the self-association and formation of micelles. One of the most interesting aspects of these
* Corresponding author. Tel.: +40-1-315-3299; fax: +40-1312-3493.
micelles is their ability to accommodate oil-soluble molecules. The increased flexibility of the micellar membrane and, as a result, the improved capability of micelles to solubilize hydrophobic molecules can be achieved by adding a coemulsifier [2]. In an ordinary o/w emulsion, the oil is located in three phases: the emulsified oil droplets, the continuous aqueous phase and the interiors of oil-swollen micelles. Once such oil droplets are present, any further oil added leads only to the formation of more or larger oil droplets, since the aqueous phase is saturated with oil. There exists a thermodynamic limit, called the ‘‘solubilization limit’’, to the amount of oil that can be taken up by the micelles. Systems with compositions below the solubilization limit are in fact examples of thermodynamically
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 2 9 - 0
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stable microemulsions, whether coemulsifier is present or not [3]. Microemulsions above the solubilization limit are kinetically stable, or more correctly miniemulsions. Systems above the solubilization limit achieve thermodynamic equilibrium by separating into two phases [4]. A knowledge of the solubilization limit of a monomer in the micellar aggregates is of considerable importance in interpreting the polymerization kinetics. The solubilization of the monomers in the surfactant–water mixture depends on the chemical structure of the components [5– 10]. Studies by NMR spectroscopy performed with ternary (emulsifier, water and monomer) mixtures revealed the fact that the monomers are distributed in the whole micellar volume [11,12]. Depending on its polarity, the monomer interacts either with the internal hydrocarbon part or with the external polar side of the surfactant [11,12]. Pyrene (Py) as a probe is particularly attractive because of its relatively long fluorescence lifetime, its ability to measure the polarity of its microenvironment, and its tendency to form excimers with a distinct fluorescence [13]. Thus, Py is a useful indicator of the polarity via the ratio of intensities of the first to the third peaks in the fluorescence emission spectrum of Py, I1 =I3 and it is widely used to probe the microenvironment of organized host media such as micelles [14]. In water, I1 =I3 is equal to 1.7, whereas in a nonpolar solvent it has a value well below 1 (e.g., in cyclohexane, I1 =I3 ¼ 0:58 [15]). In ionic (SDS) and nonionic emulsifier micelles, I1 =I3 equals 1.1–1.3, indicating that the probe environment is hydrophobic, but some contact with water still remains [16,17]. In micellar systems there is always a fraction of the probe molecules (free emulsifier molecules, oil-phase, etc.) that is in a dynamic equilibrium with that in the micellar phase. It was reported [17] that in the micellar solutions, as much as 5% of pyrene molecules remain in the aqueous phase (½Pytotal ¼ 30 lM). Furthermore, the excimer-to-monomer ratio IE/IM is a characteristic of the system that can serve as an indicator of a local concentration of a probe in the micelles. When plotted against ½Py=½E, the excimer-tomonomer ratio is a characteristic feature of a micellar system, provided that the size and aggregation number do not change. [E] represents the concentration of emulsifier. It was, however, reported that I1 =I3 increased from about 1.7 in pure water to about 2 in polyoxyethylene/ water (20/80, w/w) mixture [18]. It was concluded that that the increase in the I1 =I3 ratio upon complexation with n-dodecyl hexaethylene glycol monoether/n-hexadecyltrimethylammonium chloride mixed micelles was an indication that Py labels were solubilized in the hexa(oxyethylene) layer in the micelle. It is known that the pyrene spectrum is strongly affected by the polarity of the microenvironment [16,19]. In micellar solutions of SDS, pyrene can also be bounded to the hydrated ethylene groups close to the sulfate tails [16,19].
The polymerization kinetic order of the initiator is also dependent on the structure of the monomer in the polymerization occurring in ternary microemulsions [2,5–12,20,21]. An increase in decomposition rate of persulfate initiators was observed in ternary emulsions, ascribed to abstraction of a-proton from the substituted double bond of the monomer [22]. This effect was also found in microemulsions using alcohols as cosurfactants [23]. In the present paper, the solubilization of several monomers in the mixture water–sodium dodecylsulfate (SDS) was studied in order to find the selectivity of solute/surfactant interactions. Studies by NMR spectroscopy were performed to provide evidence for the distribution of the monomers in micellar aggregates. Fluorescence spectra were also used to gain more information on the solubilization zone of the monomers. Because it is well known that the decomposition reaction of the persulfate initiator is affected by the interaction with the monomers, the initiator decomposition and the polymerization kinetics were studied. The initiator productivity was estimated by taking into account the two simultaneous radical processes, namely the polymerization of the monomers and the initiator decomposition.
2. Experimental 2.1. Materials The monomers: vinyl acetate (VAc), acrylonitrile (ACN), ethyl acrylate (EtA), butyl acrylate (BuA), 2ethylhexyl acrylate (EHA), methyl methacrylate (MMA), butyl methacrylate (MMB), styrene (St) were commercial products and were fractionated before use. The surfactant, sodium dodecylsulfate (SDS) (>99%, Merck) and the initiator, ammonium persulfate (APS) (Loba Feinchemie) were commercial grade. 2.2. General procedure The polymerizations were performed in a glass reactor equipped with mechanical stirring (200 rpm). The reagents were added under stirring in the order: water, SDS and monomer. The mixture was degassed for 1 h with argon and heated up to 65 °C, when the initiator APS was added. Table 1 displays the concentration of the reactants. The solubilizations were performed by titration of surfactant solutions with monomers at 25 °C (Fig. 1). Due to the high viscosity of the SDS solutions, the maximum SDS/water ratio was 25/75 (weight). The solubilization capacity was calculated taking into account the amount of monomer soluble in water (Fig. 2).
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2.3. Analyses
Table 1 Nature and quantity of the reagents Reactant (g)
I
II
Monomer SDS APS H2 O Total
15 42.5 0.75 242.5 300.75
3 6 0.75 291 300.75
III 3 – 0.75 297 300.75
The 1 H-NMR spectra were recorded with a Brucker Avance DRX 400 instrument operating at 400 MHz. The chemical shifts of a-vinylic protons were determinated for 10% CDCl3 solutions (dI ) as well as in 15% SDS for D2 O solutions (dII ). The fluorescence measurements were performed with a Shimadzu RF-5001PC instrument, using pyrene
Fig. 1. Phase diagrams for SDS, water, monomers (max SDS=H2 O ¼ 25=75; 25°C).
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transparent mixtures obtained by solubilization of monomers were o/w microemulsions consisting of micellar aggregates swelled by monomers [9]. Similar one-phase microemulsion regions of monomer/water/surfactant system were obtained by other authors [5–10]. Ternary mixtures within the microemulsion region are transparent, fluid except at high SDS concentration (>25% SDS) where they are transparent and highly viscous. Using the results given in Fig. 1 we estimated the solubilization behaviour (Fig. 2). The molar ratios between solubilized monomer and surfactant were calculated by comparing the specific solubilization capacities of micelles (Fig. 2). Solubilization capacities between 0.5 and 3.5 mole monomer/mole SDS were found; the lowest values corresponding to the less polar St, EHA and the highest values for the more polar EtA and MMA. The polar monomers MMA and EtA gave the highest amount of solubilized monomer at the highest SDS concentration. The monomers presenting specific solubilization capacities independent of SDS concentration (ACN, VAc) have the highest solubility in water [26]. Furthermore, an increase in the solubilization of monomers appears with BuA and MMA. The synergistic effect of BA as a coemulsifier increased with SDS concentration [27]. For example, at the mole ratio BuA/SDS, 0.25/0.25, the coemulsifier efficiency of BuA was comparable with that of 1-pentanol (classical coemulsifier). Under the circumstances, the transparent microemulsion with the average droplet diameter 4.5 nm was formed. At lower SDS concentration, a coarse monomer emulsion or phase separation appears. The addition of styrene led to a strong increase in the droplet size. This was attributed to the different location of BA and St. It is most likely that the difference in the solubilization activity of both monomers is due to the different coemulsifier activities between St and BuA. Guo et al. [28] proved that the partitioning behaviour of styrene and 1-pentanol (POL) are different. The decreasing in POL concentration is as follows: interfacial layer oil > aqueous phase. Furthermore, the partitioning of hydrophobic styrene is modified in the
Fig. 2. The modification of the solubilization of the monomers versus SDS concentration.
(5 106 M) as fluorescence probe. The fluorescence spectra were recorded with pure monomers and with their solutions in SDS/water mixtures in concentrations similar to those used in the NMR measurements. The I1 =I3 ratios in the fluorescence spectra are given in Table 2. The monomer conversion was established gravimetrically. The APS conversion was established by ceriometric titration of the unreacted initiator [24]. The particle diameters of the latexes were determined with a NICOMP 270 instrument [25].
3. Results and discussion 3.1. Solubilization of monomers 3.1.1. Phase diagrams of microemulsions The ternary diagrams of mixtures of monomers, SDS, water are given in Fig. 1. The solubilization capacity of the monomers can be related to their polarity: the more polar EtA, MMA, VAc, ACN were solubilized in higher concentration than the less polar St, EHA, MMB. The
Table 2 Parameters of the fluorescence spectra of the pyrene in the presence of the monomers and of the microemulsions 15% SDS
MMA
EHA
BuA
CAN
St
EtA
VAc
0.807 1.019 1.132
0.92 0.931 –
1.055 1.055 –
1.137 1.087 1.038
1.273 1.078 –
2.61 (excimer) 1.611 (excimer) 1.675 (excimer)
2.931 (excimer) 1.041 1.069
1.03 1.19 1.73
1.74 1.17
1.23 1.22
0.91 2.77 3.46
2.86 8.63
3.83 (excimer) 0.84 (excimer) 1.08 (excimer)
6.9 (excimer) 6.69 5.33
I1 /I3 1.072 5% mon þ 5% SDS 10% mon þ 5% SDS 104 Intensity I1 10.22 5% mon þ 5% SDS 10% mon þ 5% SDS
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order: oil interface > water. The increased incorporation of monomer into the interfacial layer increases the solubilization capacity of the micellar aggregates. Thus, the fraction of BuA, MMA and EtA incorporated in the interfacial layer is much larger that that of styrene. Incorporation of an ‘‘amphipathic’’ additive (MMA, BuA, etc.) into the adsorbed layer of SDS around a monomer droplet reduces the electrostatic repulsion force between two SDS molecules, minimizes the oil–water interfacial tension and increases the flexibility of the interfacial layer as well as its total surface area. Under the circumstances, the spontaneous formation of transparent one-phase microemulsions can occur. Guo et al. [28] reported that the mole ratio of POL to SDS at the oil–water interface is 0.9 (the initial mole ratios: ½SDS=½POL ¼ 0:7 and ½SDS=½St ¼ 0:65). Chern and Liu [29] reported for POL 1.1 and hexanol 0.7 (with SDS), that is, the transition of macroemulsion to microemulsion starts earlier with more hydrophobic coemulsifier and the interfacial layer is more saturated with (hydrophilic) POL than with (hydrophobic) hexanol. Furthermore, the hydrophile–lipophile balance (HLB) for butanol, pentanol and hexanol was estimated to be 7, 6.53 and 6.05, respectively. The HLB data for BuA, EtA or MMA are expected to be much smaller than 6–8. Therefore, the alkyl (meth)acrylate monomers tend to adsorb into the microemulsion droplet surface layer (its inner part) or their penetration into the interfacial layer is somewhat depressed. 3.1.2. NMR spectroscopy Additional information on solubilization of monomers in SDS aggregates were obtained from 1 H-NMR spectra, observing both the signal of the surfactant and monomer. It was reported that the SDS 1 H-NMR proton spectrum has four peaks assigned SDS1, SDS2, SDS3, and SDS4 [30]:
SO4 –CH2 –CH2 –ðCH2 Þ9 –CH3 SDS1
SDS2
SDS3
SDS4
The chemical shifts of the CH3 (SDS4) group (hydrophobic head) and the methylenic protons in the CH2 OSO 3 group (SDS1) (hydrophilic site) of the surfactant were determined in the absence (d1 ) and in the presence (d2 ) of the monomer. The ratio (r) between DdCH3 =DdCH2 [11,12] calculated for the CH3 signal and corresponding dCH2 calculated for the CH2 (SDS1) signal was taken as a relative measure of the monomer location in the interfacial layer. It is represented in Fig. 3 for each monomer, at two different concentrations of monomers namely 5% and 10%. These data show each monomer located at a different domain of the interfacial layer. The highest r value obtained for hydrophobic St monomer indicates that monomer locates in the inner part of the interface (the lamelar layer of SDS). In the series of more polar monomers such as EtA, BuA, EHA
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Fig. 3. The variation of the ratio DdðCH3 Þ=DdðCH2 Þ with monomer structure for 5% and 10% weight conc. of monomers.
and MMA, MMB, a steady decrease of the r value (the shift to the polar part of the interfacial layer) was observed with increasing hydrophobicity or alkyl chain length of the alkyl ester group of the monomer and the decrease is more pronounced with alkyl acrylates: EtA 6 MMA < BuA ¼ EHA < MMB This indicates that the presence of a polar group in a monomer molecule (its hydrophilic part) favors the shift of monomer within the outer (polar) part of the interfacial layer. Furthermore, r increases with the monomer concentration due to the increased penetration of monomer into the interfacial layer (up to the saturation limit). The values of r indicate that solubilization of the above monomers occurred preferentially in the hydrophobic site of the micelles. This means that the incorporation of monomer does not screen the negatively charged ion of emulsifier, the necessary condition for the formation of thermodymanically stable microemulsion with the high fraction of solubilized oil. The hydrophilic VAc gave the lowest r value, in agreement with its shortest hydrocarbonate moiety. The value r ¼ 0:4–0:5 indicates that the domain of location of VAc within the interfacial layer is shifted to its more polar part but not to the charged part of the emulsifier. Indeed, the formation of an unstable micellar solution at higher VAc concentration does not favor the screening effect of VAc. On the contrary, the hydrophilic ACN monomer gave a very high r value. However, while all other monomers gave positive DdCH3 and DdCH2 values, ACN gave negative Dd values for both target groups. This peculiar behavior of ACN has been previously reported and was explained by its high polarity and its capacity to make loose types of micellar aggregates [11,12]. The 1 H-NMR signal of vinylic protons at the substituted carbon atom were recorded in CDCl3 with a 10% concentration of monomer (dI ) and in D2 O solution with
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Fig. 4. The differences (dII dI ) for monomers solubilized in SDS solutions (dII ) and in CDCl3 (dI ).
15% SDS for two concentrations of monomer, namely 5% and 10% (dII ). The chemical shift difference Dd ¼ dI dII is represented in Fig. 4. All signals except one for ACN moved downfield with increasing the hydrophobicity and concentration of monomer. It is well known that micelle formation, alcohol addition, and salt addition to micellar solutions of ionic emulsifiers are accompanied by downfield shift of all carbon resonances of emulsifier alkyl chains. The downfield shift is due to an increased fraction of trans conformers around the C–C bonds [31]. Elongated emulsifier chains have a greater population of trans conformers than contracted emulsifier chains. Increased aggregation of emulsifier molecules by addition of salt or alcohol causes closer packing of monomers and elongation. Thus, the downfield shift is attributed to increased incorporation of monomer into the interfacial layer due to which the shielding of vinylic protons of monomer is decreased. An increased downfield shift (Dd value) along with increasing length of the hydrocarbonate moiety of the acrylic monomers or hydrophobicity of monomer observed (EtA < BuA < EHA < St) is in good agreement with the previous results presented in Fig. 3. Elongated monomer alkyl monomer chains within the interfacial layer have a greater population of trans conformers. Thus, the conformation freedom increases from EtA to St. Polar molecules which are closer to the emulsifier polar head experience less conformation freedom (more trans conformers) than those further away. As monomer is forced away from the polar groups of emulsifier, the downfield shift appears. Thus, the least polar (or highest hydrophobic) monomer, St, gave the highest Dd value. In the case of St, the strong downfield shift (increased conformation freedom) can result from the decreased conjugation caused by the close packing of St within the lamellar phase of SDS. Furthermore, the results indicate that location of the monomer in the micellar solution
occurred at sites of lower polarity than in CDCl3 solution and these are the hydrophobic lamellar phase of the surfactant. The hydrophilic VAc monomer deviates from this trend, that is, the Dd value is relatively large. The Dd value for VAc is larger than that for BuA (the reverse was expected) and the shift is nearly independent of VAc concentration. The results indicate that VAc is located in a relatively hydrophobic environment. The independence of Dd on VAc concentration results from the saturation of interfacial layer reached already at lower monomer concentration. The high water solubility of VAc and the partitioning monomer between the micelles and the continuous phase cannot be ruled out. The fraction of the relatively hydrophilic VAc dissolved in the aqueous phase cannot be ignored and, consequently, the volume fraction of the aqueous phase decreases with increasing VAc concentration. This dilution effect together with the decreased dielectric constant of the aqueous phase influence by a complex way the micellar aggregation, solubilization and interaction between emulsifier (micelle) and monomer. Again, the ACN monomer had a different behavior: it gave a positive Dd value (upfield shift), meaning that bounding probably occurred at the polar site of SDS molecule or micelle (the hydrated tails of the surfactant) due to the decreased conformation freedom of monomer and/or the formation of (non)micellar clusters between emulsifier and ACN. Both decreased the conformation freedom of vinylic protons. This shift reveals a loosening or contraction of the monomer as the micellar solution is diluted by ACN. At very low ACN concentration the mean curvature of the emulsifier interface is toward oil since both oil (the hydrophobic chains of emulsifier) and ACN are contained within the aggregates and water is the continuous medium. A further addition of ACN can change the curvature toward to water, causing ACN (saturated with water) to become the continuous medium (the reverse micelles). This also might lead to the reverse trends (the high solubilization and upfield shift). 3.1.3. Effect of monomers on the fluorescence of pyrene Fluorescence spectra were also used to gain information on the solubilization zone, using pyrene as a fluorescence probe. The results of the fluorescence study for monomers and microemulsions are presented in Table 2. The I1 =I3 value for the aqueous solution of SDS (1.072) is in a good agreement with the literature result (1.1) [16,17]. As expected, the I1 =I3 value for organic solvents or monomers, MMA, EHA, BuA, ACN and St is smaller than that for pure water (1.7). The value of the I1 =I3 ratio obtained for St is that corresponding to aromatic solvents. The rest of the monomers present a polarity lower than that of aromatic solvents, yet higher than that for hydrocarbon solvents [16]. The I1 =I3 values
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for EtA and VAc, however, are higher than that for the pure water. This indicates that the probe is located in a very polar domain probably formed by association of monomer molecules. Similar results were observed in a polyoxyethylene (POE)/water mixture where Py was solubilized in the polar POE layer [18]. The accumulation of Py within the solvent clusters can explain the appearance of emission spectrum of the pyrene excimer. The formation of inverse micellar structures is not ruled out. The emulsification of a monomer solution of Py with the aqueous SDS solution, led to the change in the I1 =I3 value of monomer, that is, it increased with MMA, decreased with ACN and St and remained constant with BuA and EHA. Furthermore, the I1 =I3 values for solubilized MMA, EHA, BuA, ACN and St with SDS micelles are very close to that for the pure SDS micelles. This indicates that the lamellar phase formed hydrophobic alkyl chains of SDS which solubilizes both Py and monomer as well. Furthermore, the solubilized monomer seems to change slightly the environment of Py within the micellar aggregates. The hydrophobicity of the lamellar phase is lower than that of bulk styrene. This indicates that the probe environment within the micelles is hydrophobic, but some contact with water still remains [16]. The absence of an excimer indicates that Py molecules are homogeneously distributed within the micelle and shielded with hydrophobic alkyl chains and diluted by monomer. This is due to the location of both monomer and Py within the lamellar phase of SDS. The lowered I1 =I3 found with EHA is due to partitioning of monomer as well as Py between micelle shell and core or the formation of a distinct monomer core. The presence of the excimer in the spectra of a EtA microemulsions (Table 2) can be attributed to the presence of Py within the continuous phase (the monomer bulk contribution, see above) as well as in the monomer core. However, the decreased I1 =I3 value in the presence of SDS micelles results from the solubilization of a certain fraction of monomer. Furthermore, the presence of emulsifier or micelles is expected to change the original structure of the continuous phase as well as the value I1 =I3 . The accumulation of EtA as well as Py within the micelles is expected to change the I1 =I3 value close to 1.1 typical for SDS micellar systems. However, the average I1 =I3 value 1.6 indicated that the continuous phase is also operative. The experimental results show that the I1 =I3 value is nearly independent of EtA concentration. This can result from the competition between the micelle (decreases the average value) and the continuous phase (increases the average value) contributions. The appearance of a Py excimer can be attributed to the decreased solubility of Py in the VAc and therefore increased accumulation of Py molecules into the Py domains. The large I1 =I3 value (2.9) indicates a very
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polar probe environment. The solubilization of VAc in the SDS micelles led to an abrupt decrease in the I1 =I3 value. Furthermore, the excimer observed in bulk VAc phase disappeared by the addition of an aqueous phase solution of SDS. This is due to the solubilization of VAc as well as Py in the micellar phase. The I1 =I3 values 1.04– 1.07 favor the location of Py and VAc in the micellar phase. This is the reason why the excimer is missing in the fluorescence spectra obtained with solubilized VAc (Table 2). According to the foregoing data, VAc was solubilized in the polar zone and the pyrene is displaced towards the nonpolar lamellar site of SDS. Indeed, the I1 =I3 value with VAc is very close to that with pure SDS micelles. Table 2 indicates that the fluorescence intensity (I1 ) varies with the monomer type and reaction media. The largest I1 value is observed in the pure aqueous solution of SDS. The much lower I1 values are observed in the bulk monomer phase. This can be attributed to the fluorescence quenching by monomer [32,33]. The fluorescence quenching is stronger with MMA, EHA, BuA than with St, EtA and VAc. The addition of monomer to the aqueous solution of SDS led to very strong quenching and the quenching rate decreases in the following order: EtA > EHA P MMA P BuA > ACN > VAc > St The strong quenching caused by alkyl (meth)acrylate results from accumulation of Py (fluorescence probe) and quencher (monomer) at the same reaction loci (the micelle). The decreased quenching with ACN and VAc can be attributed to the different location of Py and quencher. The abrupt decrease in quenching efficiency of St in the micellar solutions may be discussed in terms of the conformation change of St within the close-packing lamellar phase of emulsifier. Furthermore, the increasing quencher (VAc) concentration slightly increases the quenching events. The reverse is true for MMA and ACN and EtA micellar solutions where the quenching decreases with increasing quencher concentration. This can be attributed to partitioning of monomer between the micelle and the aqueous phase and the shift of the quenching from the micelle to the bulk phase which is less efficient.
3.2. Relationship between polymerization kinetics and decomposition of the initiator In a previous study [23] it was shown that monomer structure influences not only the polymerization rate but also the decomposition of the initiator [22]. It was evidenced the possibility of extracting of one proton from substituted carbon atom by the persulfate initiator [22,23].
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In the present work, three different polymerization mixtures were used in order to obtain kinetic data for both homolytic reactions mentioned above (Table 1). In the first two runs, the effects of SDS concentration on the rate of polymerization and decomposition of APS were investigated (microemulsion polymerization). In run III, the polymerization and decomposition of APS were investigated under homogeneous reaction conditions in water. Fig. 5 lists the variation of amounts of both formed PVAc and decomposed APS with the reaction time. It is obvious that the initial decomposition of APS is very fast and becomes smaller at high conversion. The initial decomposition rate of APS (APS g/min) and the initial polymerization rate (g pol/min) increases in the following order: 0:002 ðg APS=minÞ ðrun IIIÞ > 0:0023 ðrun IÞ > 0:0025 ðrun IIÞ and 0:09 g=min ðrun IIIÞ < 0:16 ðrun IIÞ < 0:46 ðrun IÞ These results show that the initial decomposition rate is increased by the addition of SDS and the increase is
proportional to the SDS concentration. This is in a good agreement with the literature data where the addition of monomer, emulsifier and additives increase the decomposition rate of APS [34–37]. Tauer et al. [38] reported that the addition of St to an aqueous solution of APS strongly increased the decomposition rate of initiator. The maximum rate of APS decomposition increases in the following order (run I) (Fig. 6): MMA < EtA < BuA < EHA < SDS=water < VAc < ACN St In the run II, the maximum rate of APS decomposition increases in the following order: MMA < ACN < EtA < SDS=water VAc The similar trend is observed in pure water (run III): Water < MMA < EtA < ACN VAc The trend of APS decomposition rate (run I) is paralleled with the rate of polymerization in the SDS/alcohol/ monomer microemulsion initiated by APS; that is, the polymerization was fastest with VAc and St while slowest with MMA [24]. The compartmentalization of
Fig. 5. The variation in time of the polymerized VAc and of the decomposed APS (conds. as in Table 1) (I––42.5 g SDS, 15 g VAc; II––6 g SDS, 3 g VAc; III––0 g SDS, 3 g VAc; 0.75 g APS).
D. Donescu et al. / European Polymer Journal 38 (2002) 1691–1701
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Fig. 6. The modification of the initiator decomposition rate with the structure of the monomers (conds. as in Table 1).
reaction loci (run I) leads to the abrupt increase of the polymerization rate. The increased bimolecular termination (run III) decreases the polymerization rate. The comparison between the two simultaneous radical processes, namely the polymerization and the APS decomposition, was based on the initiator productivity P (Fig. 7). It was measured by the polymerized monomer (in moles) for 1 mole decomposed APS. In all cases, the monomer conversion was 50%. The P value was the lowest for VAc and St, the monomer inducing the most rapid decomposition of APS. On the contrary, MMA, which gave the slowest APS decomposition rate, had the highest P value (Fig. 7). This fact suggests that there is a secondary reaction consuming the initiator. Based on previous studies [22,23], these side reactions could be the extraction of the a-proton in the monomer. The MMA, lacking this a-proton, gave the highest P value. The extraction of any other protons from MMA should be much slower processes. Morris and Part [39] had shown that only perfluorurated compounds did not accelerate the decomposition of the persulfate initiator. The molecular mechanics and reactivity calculations gave the lowest energy value of the @C–Ha bond for St and VAc. At the same time, the free radicals corresponding to Ha extraction had the highest stability [23].
For estimating the Ha lability, the chemical shifts in the 1 H-NMR spectra were measured [23]. The mean d value of Ha determined in D2 O solutions with 15% SDS and 5% monomer decreased in the order: dmed ðppmÞ ¼ 7:19ðVAcÞ; 6:44ðStÞ; 6:099ðEtAÞ; 6:052ðBuAÞ; 6:033ðEHAÞ; 5:8ðACNÞ The above order corresponds to decreasing APS decomposition rate order, except for ACN. It could be explained by higher concentration of ACN in the aqueous phase (the solubilization site of APS) as well as to the increased lability of the protons in the ACN molecule due to the electronic effects of the CN group. The persulfate initiator gave higher polymerization rate and better stability of the latexes than benzoyl peroxide. These observations are in agreement with previous results in the literature [2]. A very good stability of the final mixtures obtained under conditions described above was obtained for VAc, St and EHA. The rest of the monomers gave latexes presenting particles agglomeration and phase separation in time. The mean diameter of the particles in the microlatexes obtained with St and EHA was 18 and 20 nm, respectively. The system obtained from VAc was transparent and stable; it gave precipitation on dilution in
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Fig. 7. The variation of the productivity of APS (P) for different monomers (P ¼ moles monomer polymerized=1 mole APS decomposed, conds. as in Table 1).
water. This is due to complex formation between PVAc and SDS [40,41].
4. Conclusion The solubilization of vinylic and acrylic monomers in water solutions containing SDS is strongly dependent on the nature of the monomer. Both 1 H-NMR and fluorescence studies agreed on solubilization occurring in the whole micellar aggregates. The hydrophobic monomers are solubilized towards the hydrocarbon interior of the micelles, whereas the hydrophilic ones, towards the head-group of the surfactant. The monomers induced different decomposition rates of the persulfate initiator. This fact was related to lability of the C–Ha bond in the monomer. The MMA, lacking the a-proton for double bond, gave highest productivity for the initiator decomposition.
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